1. Field of the Invention
The present invention relates generally to a method of and an apparatus for controlling the power of a natural circulation reactor and, in particular, to a method of and an apparatus for controlling the power of a natural circulation reactor both of which can readily follow up load fluctuations.
2. Description of the Related Art
Japanese Patent Examined Publication No. 43-23117 (U.S. patent application Ser. No. 497,787, filed October 19, 1965) discloses a boiling water reactor in which its in-core flow rate is controlled by control over the flow rate of feedwater, rather than the control over the in-core flow rate (control over reactor power) utilizing a recirculation system. According to the prior art, a reactor pressure vessel is arranged to cause a feedwater sparger to spray inject part of the feedwater and to inject the rest of the feedwater directly into a jet pump, the reactor pressure vessel including a heat exchanger for using feedwater supplied to the jet pump to cool feedwater supplied by the feedwater sparger and sucked by the jet pump in order to prevent cavitation in the jet pump.
Japanese Patent Examined Publication No. 42-15503 discloses a natural circulation reactor in which it is possible to adjust a natural circulation force which acts to cause natural circulation of cooling water in the reactor core. In this prior art natural circulation reactor, a large flow resistance is imparted to the bottom end of an annular cooling water descending passage defined between a reactor pressure vessel and a cylindrical shroud which surrounds the reactor core, whereby the water level of cooling water in the annular cooling water descending passage is maintained at a position higher than the water level of cooling water in the shroud. Adjustment of the natural circulation force in such a reactor can be effected by setting the flow resistance at a suitable level as well as by introducing cooling water into the annual cooling water descending passage at a suitable flow rate.
However, the boiling water reactor described in Japanese Patent Examined Publication No. 43-23117 requires that the heat exchanger be incorporated in the reactor pressure vessel in order to adjust the in-core flow rate, and the internal structure of the reactor pressure vessel inevitably becomes complicated. The heat exchange effectiveness of the heat exchanger is low and the formation of cavitation in the jet pump can only be prevented within a narrow range because of a small temperature difference (about 20.degree. C.) between the feedwater which is directly injected into the jet pump and cooling water within the reactor pressure vessel which is sucked into the jet pump by the action of the directly injected feedwater.
The prior art natural circulation reactor described in Japanese Patent Examined Publication No. 42-15503 has the capability to realize the natural circulation force which acts to cause natural circulation of cooling water in the reactor core, without involving the problems with the reactor disclosed in Japanese Patent Examined Publication No. 43-23117. However, in this prior art natural circulation reactor, the cooling water is heated in the reactor core and converted into steam, the steam being introduced into the annular cooling water passage in which the steam is condensed into a liquid by heat exchange with water which serves as a secondary fluid in the heat exchanger incorporated in the annular cooling water passage, and the thus-obtained liquid being again supplied to the reactor core through the bottom portion thereof. This circulation passing through the reactor core is realized by a natural recirculation force. Prior to the operation of the reactor, the natural recirculation force is adjusted as described previously by setting the flow resistance at a suitable level and by introducing cooling water into the annular cooling water descending passage at a suitable flow rate.
It has recently been desired that such a natural circulation reactor be provided with the capability to follow load fluctuations. The above-described natural circulation reactor of the prior art, however, a load-follow-up operation is impossible since the in-core flow rate during the operation of the reactor cannot be adjusted. Therefore, this type of natural circulation reactor is arranged so that control over reactor power can only be realized by the operation of control rods.